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The Importance of Zeta Potential Measurements & Role of Ionic Strength in Flocculation Processes

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Published in Water and Technology. 2013, vol. 4, no. 1, p. 1-5
Abstract For efficient water clarification, flocculation is usually induced using positively charged polyelectrolytes such as synthetic polymers to rapidly separate the liquid phase from the solid one and obtain a clear filtrate. Generally, the use of polymeric flocculants over inorganic polyelectrolytes, such as poly-aluminum complexes, gives significant advantages when the water has a high concentration of suspended solids; the concentration of the polymeric flocculant is lower, the resulting sludge is more compact, and there is less coagulant left in the water after treatment. However polymeric flocculants are not always used in a rational way for optimal flocculation regarding the natural fluctuations and heterogeneity of water composition (e.g., pH, ionic composition), suspended particle concentration and corresponding physicochemical properties (e.g., sizes, shapes, surface charges). An improved understanding of the interaction mechanisms between flocculants and the suspended material is also often necessary. Since most of the flocculation mechanisms are mainly driven by electrostatic interactions, the polymer and aggregate electrostatic charges are key parameters to take into consideration. There are different manners to determine the polymers or flocs charge; the first one is considering the chemical charge amount, usually determined by potentiometric titrations . The second is considering the effective charge at the slipping plane of the diffuse layer of the flocs. This parameter is usually determined by electrophoretic measurements, and this charge is commonly referred to the zeta potential. Zeta potential is a convenient way to optimise flocculant dosage in water and wastewater treatment since the key to effective particle removal is the reduction of their zeta potential. The ability of modern methods and improvements in reliability of measuring techniques has reinvigorated the “on site” application of zeta potential and extensive characterization of water. The use of charged synthetic polymers involves different and often concomitant flocculation mechanisms. When the polymer radius of gyration is small in comparison to the particle size, flocculation is mainly promoted by local polymer adsorption and local inversion of the sign of the surface charge. Thus, oppositely charged patches are then created at the particle surfaces which can interact with oppositely charged surfaces. Depending on the flocculant dosage, complete charge neutralization can be achieved so that van der Waals forces, which are always attractive, induce particle flocculation. However, if flocculant dosage is not properly controlled, an excess of polymers will redisperse the particle suspension by charge inversion or steric stabilization. Usually, from a practical point of view, three different regimes must be considered: (1) when flocculant concentration is too low to induce rapid flocculation, (2) flocculant dosage is optimal for rapid destabilization, and (3) flocculant concentration is too high and results in particle restabilization. The floculant branching also plays here a key role since it is largely controlling the polymer interaction behavior with the particles. Moreover, the branching also induces important differences in the floc structure, compactness, and fractal dimensions, which are affecting the sedimentation rates and floc strength with regards to filtration processes. Among the aforementioned factors, there is also an important one which concerns the potential impact of the solution ionic strength due to the presence of dissolved ions, simple or multivalent electrolyte (salt) in the solution. Owing to the non linear effects of an increase (or decrease) of the solution ionic strength, salt influence is not trivial to estimate. Indeed, salt is expected to screen the electrostatic repulsive forces between the charged particles in solution and thus via short-range van der Waals interactions to promote the rate of particle flocculation. On the other hand, is it also expected by screening effects to reduce the electro-attractive interactions between the charged flocculants and the oppositely charged particles and thus strongly affect the flocculation rate and polymer efficiency as shown in figure 1. The presence of salt is also expected to play a role on the flocculant conformations by promoting, for example, gel formation, flocculant folding and collapse. In the study and examples presented here, positively charged linear chains as well as positively charged branched polymers having two branches are used as flocculants. The influence of the polyelectrolyte branching and polymer dosage on the kinetics of flocculation and changes of the electrostatic properties of well characterized micrometric particles are addressed for the sake of clarity. The polymer efficiency is evaluated by considering the flocculation rates of solutions containing well-defined negatively charged and monodisperse latex particles. The impact of the presence of salt at various concentrations is also investigated at optimal flocculation concentration dosage, and empirical kinetics constants are determined by adjusting the salt concentration. The electrokinetic properties of the particles are calculated to obtain an insight into the possible parameters governing the flocculation processes and polymer efficiency. Flocculation rates, optimal polymer dosage, and stability ratio, which are key topics in the colloid and polymer science, are compared with salt-induced particle destabilization only to elucidate the impact of the ionic strength influence on the flocculant efficiency.
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Structures
Research groups Environmental Physical Chemistry
ISE Pôle Sciences
ISE Eau
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STOLL, Serge. The Importance of Zeta Potential Measurements & Role of Ionic Strength in Flocculation Processes. In: Water and Technology, 2013, vol. 4, n° 1, p. 1-5. https://archive-ouverte.unige.ch/unige:26837

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Deposited on : 2013-03-18

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